1. Field of the Invention
The invention relates to a seal assembly, for example, in an expansion joint, for use in a subterranean well in an environment hostile to common elastomers.
2. Description of the Prior Art
A common sealing member in subterranean well tools is a nitrile rubber O-ring or chevron seal. A typical application of such a sealing member is in a tubing string expansion joint. Owing to temperature changes and pressure changes along the tubing string, the length of the tubing string varies. When the lower end of the string is anchored in the well casing, as by a packer, or a casing bore receptacle, an expansion joint is provided to compensate for the changes in tubing string length to avoid excessive forces and buckling of the tubing. An expansion joint commonly comprises two telescoping sleeve members sealed by annular, elastomeric sealing elements between the telescoping members. The sealing elements are normally maintained in compression, whereby the elastomeric property of the sealing elements maintains sealing pressure against the sealing surfaces of the telescoping members, even with temperature changes and pressure cycling.
The environment in many deep gas wells is characterized by the presence of high temperatures, carbon dioxide, and hydrogen sulfide. Common elastomers such as nitrile rubber will rapidly deteriorate in such an environment. Furthermore, elastomeric materials often take a "compression set", particularly in these environments. In dynamic seal applications, this set results in a loss of elasticity with time, which will result in leaks with pressure and temperature cycling. Such elastomeric materials are thus made effectively non-elastomeric at the time of, or during, sealing, because of such compressive set. Non-elastomeric materials such polytetrafluoroethylene, known by the trademark Teflon, are known as sealing materials having resistance to such environments. Such materials, however, have not been used alone in dynamic seals in subterranean wells because there are not self-energizing, that is, sealing pressure cannot be maintained merely by elastomeric compression and expansion. Furthermore, after the loss of sealing material, by extrusion for example, elastomers expand to maintain pressured sealing contact, while non-elastomers normally do not.